Recently, energy storage technology has been given increasing attentions. Efforts into research and development for electrochemical devices have been actualized more and more, as the application of energy storage technology has been extended to energy for cellular phones, camcorders and notebook PC and even to energy for electric vehicles. In this context, electrochemical devices have been most spotlighted. Among such electrochemical devices, development of rechargeable secondary batteries has been focused.
Among the commercially available secondary batteries, lithium secondary batteries developed in the early 1990's have been spotlighted, since they have a higher driving voltage and significantly higher energy density as compared to conventional batteries, such as Ni-MH batteries, N—Cd batteries and sulfuric acid-lead batteries using an aqueous electrolyte.
In addition, such lithium secondary batteries have been manufactured to have a large surface area for the purpose of realizing high capacity and high density. To improve electroconductivity, a conductive material has been added to an electrode active material. However, there have been problems in that such a conductive material cannot be dispersed homogeneously or side reactions occur after using batteries repeatedly. To solve the above-mentioned problems, it has been suggested that a conductive material coating layer is formed on at least one surface of a negative electrode, where side reactions mainly occur, to inhibit generation of side reactions. However, there has been a problem in that the thickness of an electrode assembly is increased due to such a conductive material coating layer, resulting in an increase in resistance.
To form such a conductive material coating layer, the following two methods have been used frequently: a method applying and drying slurry containing a negative electrode active material to one surface of a negative electrode current collector and further applying slurry for forming a conductive material coating layer (also referred to as conductive material slurry hereinafter); and a method applying conductive material slurry to negative electrode active material slurry while carrying out a drying step at the same time.
However, the above-mentioned methods are problematic in that the two-step process requires carrying out coating steps twice and thus causes degradation of productivity and the preliminarily coated negative electrode active material is separated during a conveying process for the subsequent conductive material coating step, while the process of applying conductive material slurry at the same time causes mixing of the negative electrode active material slurry with the conductive material slurry so that the conductive material coating layer may not be controlled.